Conventionally, this type of waveguide type optical circuit element includes, as a principal part thereof, a directional coupler as schematically shown in FIG. 10. In this figure, a conventional waveguide type optical circuit element, such as a polarity-independent optical switch, has a lithium niobate (LiNbO.sub.3 hereinafter) substrate 1 defining two waveguides 2 and 3, parts of the waveguides 2 and 3 being close to each other to form a coupling section 6. When light of intensity PO enters one of the waveguides 2 from a left incident end surface, it varies depending on coupling length L of the coupling section 6 where the parts are close to each other, and lights of varied intensities PA, PB exit the respective waveguides 2 and 3 from a right exit end surface. The first length l for the exit lights of intensities PA, PB to be in the relationship PA/(PA+PB).apprxeq.0 is called a complete coupling length. FIG. 11(A) shows the relationship of exit lights PA, PB with the complete coupling length l. Generally, the complete coupling length l varies according to the polarization state of incident light, namely whether the TE mode or TM mode. Here TE mode refers to a polarization state in which electric field components are parallel to the substrate 1, and TM mode refers to a polarization state in which the electric field components are vertical to the substrate 1. It is to be noted that the two waveguides 2 and 3 have an identical structure.
The conventional directional coupler of the polarity-independent optical switch acting as a waveguide type optical circuit element includes a pair of electrodes (not shown) in the coupling section 6 to be the uniform .DELTA..beta. type, and is capable of switching incident light in accordance with the electro-optical effect produced by applying an electric field. This switching state is shown in FIG. 11(B) as a uniform .DELTA..beta. switching diagram. In this figure, state (bar state) indicates a state where incident light intensity PO corresponds to exit light intensity PA, with the other exit light intensity PB=0, and state (cross state) indicates a state where incident light intensity PO corresponds to exit light intensity PB, with the other exit light intensity PA=0. The state appears on a plurality of circular arcs when an electric field (.DELTA..beta..multidot.L/.pi.) is applied.
Next, FIG. 12 shows switching diagrams of both the TE and TM modes where an electric field is applied with the ratio L/l between coupling section length L and complete coupling length l is "1" (L/=1) for both the TM mode and TE mode. In this figure, as the electric field is applied to increase .DELTA..beta..multidot.L/.pi., the TE mode becomes state at .DELTA..beta..multidot.L/.pi..apprxeq.5.2 and, as the electric field is applied further to increase .DELTA..beta..multidot.L/.pi., the TM mode becomes state at .DELTA..beta..multidot.L/.pi..apprxeq.5.9. In this way, switching of incident light is made by changing both the TE and TM modes from state to state.
Another conventional polarity-independent optical switch comprising a waveguide type optical circuit element is described in ELECTRONICS LETTERS Oct. 8, 1987 Vol. 23, No. 21, pages 1167-1168, which is shown in FIG. 13. In this figure, the conventional polarity-independent optical switch has a LiNbO.sub.3 substrate 1 defining two waveguides 2 and 3, a pair of electrodes 4 and 5 arranged on the two waveguides 2 and 3, and another pair of electrodes 45 and 55 arranged outwardly of the pair of electrodes 4 and 5.
In the above construction, the condition for both the TE and TM modes to become state with the same voltage applied to the electrodes 4, 5, 45 and 55 is determined by electro-optical coefficients .gamma.13 and .gamma.33 of the LiNbO.sub.3 substrate 1. FIG. 14 shows the crosstalk of the TE and TM modes for .DELTA..beta. (corresponding to the applied voltage) in this case. In this figure, .alpha.=.DELTA..beta.TE/.DELTA..beta.TM(.DELTA..beta.TE, .DELTA..beta.TM being phase mismatches of the directional coupler for the TE and TM modes, respectively). When 0.25.ltoreq..alpha..ltoreq.0.34, the crosstalk can be made -20 dB or less for both modes by adjusting the applied voltage. The actual value in the vicinity of 1.3 .mu.m wavelength is .alpha.=0.29 which satifies the above condition, and therefore the voltage for producing state may be equalized for both the TE and TM modes.
Still another conventional polarity-independent optical switch comprising a waveguide type optical circuit element is described in ELECTRONICS LETTERS Sept. 15, 1988 Vol. 24, No. 19, pages 1198-1200. This conventional polarity-independent optical switch secures the polarity independence by using an X-cut crystal which produces the electro-optical effect equally for both the TE and TM modes in the case of an optical crystal such as LiNbO.sub.3.
Since the conventional waveguide type optical circuit element is constructed as noted above, .DELTA..beta..multidot.L/.pi. for mode switching is different between the TE mode and TM mode as shown in the switching diagram of FIG. 12, which has the problem that polarity independence cannot be made perfect. In another conventional waveguide type optical circuit element, an equal complete coupling length for the TE and TM modes is obtained relatively close to a cut-off condition. However, since the optical waveguides are thin and produces a weak confinement effect, waveguide propagation loss and radiation loss at curved portions of the waveguides tend to be large. Thus, there is the problem of requiring a measure such as for increasing the waveguide width at the curved portions. The waveguide type optical circuit element using the X-cut crystal has a weak electro-optical effect available, and therefore has the problem of necessitating a high drive voltage.
This invention has been made to solve such problems, and its object is to provide a waveguide type optical circuit element which has reduced polarity dependence, provides waveguides that have excellent light confining effect, and requires a reduced drive voltage.